Various embodiments relate to the operating and dimming of light sources which include a multiplicity of LEDs; however, it can also be used for operating and dimming individual LEDs. In the case of light sources which include a plurality of LEDs, the light color and/or the brightness of the individual LEDs can be very different during the dimming, owing to variation in the parameters of the individual LEDs (manufacturing variation), in particular owing to the variation in the individual non-linear characteristic curves of the LEDs. In the case of currents which are a fraction of the nominal current, the behavior of most LEDs is therefore not precisely specified.
The switching converters which are generally used to drive LEDs, for instance those disclosed in the document US 2006/0033484A1, are therefore driven using a radio-frequency PWM signal, wherein the duty ratio of said RF signal is reduced for dimming.
In document US 2008/0224629A1, the radio-frequency PWM signal of a switching converter in the OFF state of a connected phase-control or phase-gating dimmer is operated in a mode which damps oscillations between switching converter and dimmer.
In document US 2012/0019160A1, the amplitude of a low-frequency PWM current is adjusted by means of a series regulator on the basis of the duty cycle of the corresponding low-frequency PWM signal.
In the case of all of the abovementioned conventional dimming methods, the current supplied to the LEDs during the ON times must not fall below a predefinable minimum, since otherwise the abovementioned negative effects would arise. Usually, said minimum is assumed to be at approximately 30% of the nominal current. In order to achieve dimming values which are below said predefinable dimming threshold value, the conventional RF signal, which signal corresponds to a current through the LEDs of approximately 30% of the nominal current, is gated, that is to say a low-frequency PWM signal is superposed thereon, with the result that the RF signal is only forwarded to the output of the drive apparatus during certain times and not the rest of the time (so-called PWM dimming).
Even if it were possible for the low-frequency PWM dimming to be performed already on the basis of the nominal current, this is not done in practice owing to the noise (inductor) which occurs as a result.
In this connection, FIG. 1 shows the temporal profile of the drive signal AS for two different dimming values, said signal being provided at the converter switch by the drive device. A low-frequency PWM signal is superposed on said RF signal, which has a period T1 and an ON time ton1 and an OFF time toff1, said low-frequency PWM signal virtually gating the RF signal. In the schematic illustration from FIG. 1, the PWM signal has the ON time ton2 and the OFF time toff2 and a period T2 before the dimming step. After the dimming step, the period is unchanged, that is to say T3=T2, but the ON time ton3 has reduced compared with ton2 by a period T1 of the RF signal; toff3 has increased correspondingly compared with toff2. FIG. 1 also shows the respective output currents Iout3 and Iout2 of the converter which occur in both cases.
The RF signal is, for example, between 60 kHz and 150 kHz, while the PWM signal can be, for example, between 100 Hz and 1000 Hz. Owing to this frequency ratio, it is obvious that only discrete dimming steps can be adjusted, that is to say only a whole period T1 of the RF signal can be chopped from one dimming value to the next-lowest dimming value. Owing to tolerances, whether they be caused by fluctuations in temperature or the supply voltage, it is not possible in practice to set ton3 such that it becomes located at a desired point within ton1, in order to obtain a finer dimming resolution as a result. By means of the illustrated process, it is always whole periods of the RF signal that are chopped when changing the dimming setting.
As long as the dimming setting does not yet represent very low dimming values, this has a barely visible effect. If, for example, 500 periods T1 are contained in ton2, then a much smaller dimming step can be achieved by chopping one period of the RF signal, that is to say that ton3 has 499 periods of the RF signal. In the case of low dimming values, however, the chopping of one period of the RF signal leads to large jumps in terms of percentage in the light emitted by the LEDs. If, for example, only 10 RF periods T1 are present and one is chopped, this corresponds to a jump of 10%.
Said jumps are visible in the output current and can have a disturbing effect. A finer resolution is not possible in the case of low dimming values. Moreover, the frequency of the RF signal can change (regulated system) owing to temperature or voltage changes in the system. The PWM signal for dimming does not fit in owing to a separate generation thereof. If a period of the RF signal continually disappears and reappears owing to said oscillations (limit oscillation), an undesirable flickering can result.